Reflective alloy film

ABSTRACT

A reflective alloy film having high strength, high corrosion resistance, and high reflectivity includes 0.01-2.0 at % of at least one strengthening element, 0.01-2.0 at % of at least one anti-corrosive element, 0.01-3.0 at % of at least one reflective element, and a balance amount of silver. The total atomic percentage of the strengthening element, the anti-corrosive element, and the reflective element is less than 5.0. The reflective alloy film is useful as a reflective layer or a semi-reflective layer for various electronic devices, such as flat panel displays and optical storage media.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a reflective alloy film, more particularly to a reflective alloy film having high strength, high corrosion resistance, and high reflectivity.

2. Description of the Related Art

A metal film has been widely applied in various electronic products to enhance or improve original properties of a substrate and to meet specific requirements of the products. Since silver has high reflectivity, a film made of pure silver can meet the general requirement of mirror-like reflection for a metal film. Therefore, silver is a primary material used in the manufacture of a reflective film.

In order to reduce the manufacturing cost by minimizing the amount of silver used in the film or to provide the film with desirable transmittance or reflectivity, the film is usually formed on a substrate by vapor depositing silver on the substrate so as to reduce the thickness of the silver film and thus the amount of silver. However, the reduction in the thickness of the silver film formed by vapor deposition decreases the strength and the corrosion resistance of the film, which causes the film to oxidize and to deform in an atmospheric environment, especially at an elevated temperature or in a humid condition.

Therefore, various silver alloys, each of which contains selective additives, have been developed to improve the properties of the silver film.

Various silver alloys containing palladium, copper, tin, indium, gallium, or zinc therein can enhance strength and hardness while having a high compatibility with ceramic substrate (see, for example, the “Noble Metal” section in “Metals Handbook” published by American Society for Metals).

Various silver alloys with selectively added copper, neodymium, tin, platinum, palladium, or zinc therein can achieve corrosion resistance, and retard the undesirable oxidation. (see, for example, Japanese Patent Nos. 2000-395894, 2002-15464 and 2003-160827, and Taiwanese Patent Nos. 514,909 and 531,562).

Various silver alloys with selectively added platinum, palladium, copper or gold therein can minimize the reduction of reflectivity. Since such silver alloys have a reflectivity substantially identical to that of pure silver, they can be used in applications such as CD-R, writable DVD disk, and the like (see, for example, U.S. Pat. Nos. 5,948,497 and 6,007,889).

Nevertheless, in view of the development of high storage capacity and high reading and/or writing speeds for electronic products, it is desirable to provide the reflective film with high strength, high corrosion resistance, and high reflectivity.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a reflective alloy film having high strength, high corrosion resistance, and high reflectivity.

The reflective alloy film according to this invention includes 0.01-2.0 at % of at least one strengthening element, 0.01-2.0 at % of at least one anti-corrosive element, 0.01-3.0 at % of at least one reflective element, and a balance amount of silver. The total atomic percentage of the strengthening element, the anti-corrosive element, and the reflective element is less than 5.0.

The strengthening element decreases the grain size of the alloy film so as to increase the mechanical strength of the film. The anti-corrosive element can form a dense oxide film on the alloy film so as to protect the alloy film from corrosion. The reflective element enlarges area of a solid solution of the alloy film so as to prevent formation of precipitates and to result in high reflectivity.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments with reference to the accompanying drawings, of which:

FIG. 1 is a transmission electron microscopic photograph of a preferred embodiment of the reflective alloy film according to this invention illustrating a microscopic structure of the alloy film containing 0.5 at % titanium, 0.05 at % indium and 0.5 at % copper; and

FIG. 2 is a transmission electron microscopic photograph of a comparative film illustrating a microscopic structure of the film made of pure silver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiment of the reflective alloy film according to this invention contains at least one strengthening element, at least one anti-corrosive element, at least one reflective element, and silver.

The reflectivity of the silver alloy is influenced by the existence of a second phase element. In the present invention, at least one of the highly reflective elements, which is selected from the group consisting of palladium, copper and platinum, is added to silver under the condition that reflectivity of the silver alloy does not decrease significantly. Since the reflective element, such as palladium, copper, or platinum, has a face-centered cubic structure, like silver, it is compatible with silver, and thus can enlarge the single phase area of the silver alloy phase diagram. As a result, precipitation can be avoided, and the scattering phenomenon and the reduction of reflectivity due to the precipitates can be minimized. As shown in Table 1, although the addition of the reflective element reduces the reflectivity of silver, the reflectivity reduction is not significant. Preferably, the amount of the reflective element ranges from 0.01 at % to 3.0 at %. If the amount of the reflective element is above 3.0 at %, the reflectivity of the alloy film will decrease significantly as the amount of the reflective element is increased. TABLE 1 Reflectivity 200 nm thickness, reflectivity(%) Composition of film at 650 nm wavelength Ag 98 Ag—2.0at % Pd 98 Ag—2.0at % Cu 97 Ag—2.0at % Pt 96

The anti-corrosive element can form a dense oxide film on the alloy film so as to enhance the corrosion resistance of the alloy film. As shown in Table 2, the anti-corrosive element is preferably scandium, beryllium, aluminum, titanium, chromium, zinc, nickel, or mixtures thereof. Since the anti-corrosive element has a high reactivity with oxygen, the anti-corrosive element added to the alloy film will migrate to the surface of the alloy film so as to form a dense anti-corrosive oxide film thereon. Preferably, the amount of the anti-corrosive element ranges from 0.01 at % to 2.0 at %. If the amount of the anti-corrosive element is above 2.0 at %, the reflectivity of the alloy film may be adversely affected thereby. TABLE 2 Anti-corrosive element Anti-corrosive Oxidation free energy (kJ/mole element oxygen) at 127° C. Scandium(Sc) −596 Beryllium(Be) −570 Aluminum(Al) −511 Titanium(Ti) −468 Chromium(Cr) −342 Zinc(Zn) −310 Nickel(Ni) −202

In order to strengthen the film and to maintain a single phase microstructure of the silver alloy, the principle of solid solution strengthening mechanism is applied in the present invention. As shown in Table 3, the strengthening element is selected from barium, scandium, silicon, titanium, indium, germanium, zinc, bismuth, and mixtures thereof. The atomic radiuses of these strengthening elements all have 5% or higher difference from the atomic radius of pure silver. The dislocation migration in the atomic lattice of these alloys is reduced by elastic strain of the atomic lattice. Therefore, the grain size is reduced, and the strength of the alloy film is increased. Preferably, the amount of the strengthening element is controlled to be in a range from 0.01 at % to 2.0 at %. TABLE 3 Strengthening element Radius difference as compared to silver Barium(Ba) 59% Scandium(Sc) 19% Silicon(Si) 17% Titanium(Ti) 14% Indium(In) 14% Germanium(Ge) 13% Zinc(Zn) 13% Bismuth(Bi) 7%

In Table 4, examples of this invention and comparative examples are compared in terms of reflectivity, relative mechanical strength, and relative corrosion resistance. The comparative examples include a film of pure silver, a silver alloy film containing 2 at % copper, a silver alloy film containing 2 at % platinum, and a silver alloy film containing 1 at % neodymium. The examples of this invention include a silver alloy film containing 0.5 at % germanium, 0.5 at % nickel, and 1.0 at % platinum (referred as Example 1 hereinafter), and a silver alloy film containing 0.5 at % titanium, 0.05 at % indium, and 0.5 at % copper (referred as Example 2 hereinafter).

The relative mechanical strength is determined on a basis of grain size of a sputtered film. Since the mechanical strength is inversely proportional to the square root of the grain size, the smaller the grain size, the higher will be the relative mechanical strength. Four numbers from 1 to 4 are used to evaluate the mechanical strength. The smaller the number, the lower is the relative mechanical strength.

The relative corrosion resistance is determined on a basis of the duration of delaying an abrupt reduction of the reflectivity at constant temperature under constant relative humidity. The longer the duration, the better is the corrosion resistance. The relative corrosion resistance is evaluated using numbers from 1 to 4. The smaller the number, the worse is the corrosion resistance. TABLE 4 comparison of properties between the examples of this invention and the comparative examples Relative Relative Reflectivity mechanical corrosion Composition at 650 nm strength resistance Comparative Ag 98% 1 1 Ag—2at%Cu 97% 2 2 Ag—2at%Pt 96% 3 4 Ag—1at%Nd 90% 4 3 Examples Example 1 96% 3.5 4 (Ag—0.5at% Ge—0.5at%Ni— 1.0at%Pt) Example 2 97% 3 3.5 (Ag—0.5at% Ti—0.05at%In— 0.5at%Cu)

As shown in Table 4, generally, Examples 1 and 2 have significantly improved properties in connection with mechanical strength and corrosion resistance while attaining a reflectivity above 95%.

The alloy film of Example 1 has a reflectivity of 96%, which is comparable to that of the comparative alloy film containing 2 at % platinum. However, the addition of germanium (used as the strengthening element) and nickel (used as the anti-corrosive element) improves the mechanical strength and the corrosion resistance of the alloy film of Example 1 over the comparative example. Furthermore, the alloy film of Example 1 has a reflectivity superior to that of the comparative example containing 2 at % neodymium.

The alloy film of Example 2 has a reflectivity of 97%, and has the mechanical strength and the corrosion resistance superior to those of the comparative alloy film containing 2 at % copper, which has a reflectivity identical to that of Example 2.

In view of the results shown in Table 4, the alloy film of this invention possesses high strength, high corrosion resistance, and high reflectivity. Therefore, the alloy film of this invention is useful as a reflective film for various electronic products, such as flat panel displays and optical storage media.

FIGS. 1 and 2 show transmission electron microscopic photographs of the alloy film of Example 2 and the comparative film made of pure silver. The alloy film of Example 2 has a grain size less than 10 nm, and the grains thereof are distributed densely. Therefore, as compared to the silver film shown in FIG. 2, which has a grain size of about 100 nm, the grain size of the alloy film of Example 2 is reduced significantly, which in turn demonstrates that the mechanical strength of the alloy film of Example 2 is superior to that of the comparative silver film. It was further confirmed by electron diffraction pattern that the atomic structure of the alloy film of Example 2 is a single phase face-centered cubic structure.

In fact, when the alloy film of this invention is made in a thickness of 100 nm, it not only has a reflectivity of 97% at a wavelength of 650 nm, which is comparable to that of silver film (98%), but also passed a corrosion resistance test at a temperature of 80° C. in a relative humidity of 85% (The duration till the reduction in the reflectivity is over 100 hrs). Therefore, the alloy film can be used as a full-reflective layer of a high-speed write-once DVD disk.

When the alloy film of this invention is made in a thickness less than 20 nm, it still has a reflectivity above 25%, while passing the corrosion resistance test at a temperature of 80° C. in a relative humidity of 85% (The duration till the reduction in the reflectivity is longer than 96 hrs). Therefore, the alloy film can be used as a semi-reflective layer of a double-layer read-only recording medium or a double-layer write-once recording medium, or as a reflective layer of a reflective/semi-reflective flat panel display.

While the present invention has been described in connection with what is considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation so as to encompass all such modifications and equivalent arrangements. 

1. A reflective alloy film comprising: 0.01-2.0 at % of at least one strengthening element; 0.01-2.0 at % of at least one anti-corrosive element; 0.01-3.0 at % of at least one reflective element; and a balance amount of silver, the total atomic percentage of said strengthening element, said anti-corrosive element, and said reflective element being less than 5.0.
 2. The reflective alloy film as claimed in claim 1, wherein said strengthening element is selected from the group consisting of barium, scandium, silicon, titanium, indium, germanium, zinc, and bismuth.
 3. The reflective alloy film as claimed in claim 1, wherein said anti-corrosive element is selected from the group consisting of scandium, beryllium, aluminum, titanium, chromium, zinc, and nickel.
 4. The reflective alloy film as claimed in claim 1, wherein said reflective element is selected from the group consisting of palladium, copper, and platinum.
 5. The reflective alloy film as claimed in claim 1, wherein said strengthening element is germanium, said anti-corrosive element is nickel, and said reflective element is platinum.
 6. The reflective alloy film as claimed in claim 5, comprising 0.5 at % of said germanium, 0.5 at % of said nickel, and 1.0 at % of said platinum.
 7. The reflective alloy film as claimed in claim 1, wherein said strengthening element is indium, said anti-corrosive element is titanium, and said reflective element is copper.
 8. The reflective alloy film as claimed in claim 7, comprising 0.05 at % of said indium, 0.5 at % of said titanium, and 0.5 at % of said copper. 